Zeolite can be used to soften water when

• A

  ${\mathrm{Ca}}^{2+}\text{ or }{\mathrm{Mg}}^{2+}\text{ ions in zeolite are replaced by }{\mathrm{Na}}^{+}\text{ions in hard water }$

• B

  ${\mathrm{Ca}}^{2+}\text{ ions in zeolite are replaced by }{\mathrm{Mg}}^{2+}\text{ ions in hard water }$

• C

  ${\mathrm{Na}}^{+}\text{ions in zeolite are replaced by }{\mathrm{Ca}}^{2+}\text{ and }{\mathrm{Mg}}^{2+}\text{ ions in hard water }$

• D

  None of the above